* Grains and soybeans fall, attention on large supplies

* Stronger dollar burdens U.S. export prospects

HAMBURG, Feb 26 (Reuters) - Chicago corn, wheat and soybeans fell on Monday, giving up early gains as focus returned to large world supplies and expectations of bumper South American harvests which last week pushed corn to its lowest in around three years.

A firmer dollar, making U.S. grains more expensive in export markets, also weighed.

Chicago Board of Trade most-active corn was down 0.7% at $4.10-1/4 a bushel by 1116 GMT, having dropped to its lowest since November 2020 last week. Lesser-traded March corn on Monday was below the $4 level at $3.96-1/4 a bushel.

Soybeans fell 0.2% to $11.38-3/4 a bushel, wheat fell 1.3% to $5.61-3/4 a bushel.

Large global supplies of corn, wheat and soybeans and tough competition for U.S. supplies in export markets recently weakened prices.

Improved crop prospects for corn in Brazil and Argentina on top of bumper U.S. harvests last year helped to drive spot CBOT corn futures down.

“With such hefty South American supplies set to seek export sales the debate is now about whether corn can hold the $4 level,” one European trader said.

Traders said soybeans faced headwinds as the focus returned to slack U.S. exports and news of Brazilian soybean export shipments to the U.S.

Ample global supplies of wheat and low prices from Russia and other Black Sea exporters also pressured.

“There is hope low corn and wheat prices will stimulate more import demand, with South Korea buying wheat from the U.S., Canada and Australia,” one German trader said.

“The question now is whether the big wheat importers like Egypt, Algeria and Saudi Arabia will return with new purchase tenders this week. But Russian and other Black Sea region prices are still about lowest which means the Black Sea would be the favourite to win new business this week.” (Reporting by Michael Hogan in Hamburg, additional reporting by Naveen Thukral in Singapore; Editing by Subhranshu Sahu, Janane Venkatraman and Kirsten Donovan)